<!DOCTYPE html>
<html lang="en">
    <head>
        <meta charset="utf-8"/>
        <title>0000</title>
        <style>
            .box {
                width:100px;
                height:100px;
                background:red;
                transition:1s 3s width;   <!--第二个秒数是延迟时间-->
            }
            .box:hover {
                width:800px;
                height:200px;
                background:blue;
            }
        </style>
    </head>
    <body>
        <div class="box"></div>
    </body>
</html>